Contact lenses are widely used for correcting many different types of vision deficiencies. These include defects such as near-sightedness and far-sightedness (myopia and hypermetropia, respectively), astigmatism, and defects in near range vision usually associated with aging (presbyopia).
Astigmatism occurs as the refractive error in an eye is dependent upon meridian. This is usually due to one or more refractive surfaces, most commonly the anterior cornea, having a toroidal shape. It may also be due to one or more surfaces being transversely displaced or tilted. Astigmatism is usually regular, which means that the principal (maximum and minimum power) meridians are perpendicular to each other. People with astigmatism have blurred vision at all distances, although this may be worse at distance or near, depending on the type of astigmatism. These people may complain of sore eyes and headaches associated with demanding visual tasks. Astigmatism can corrected with an astigmatic ophthalmic lens, which usually has one spherical surface and one toroidal (cylindrical) surface.
It is believed that presbyopia occurs as a person ages when the lens of eye loses its elasticity, eventually resulting in the eye losing the ability to focus at near distances (when distance vision is corrected), such as the normal reading distance, and in some cases at intermediate distances. Presbyopic persons (presbyopes) complain of difficulty performing close tasks. To compensate for presbyopia, ophthalmic lenses are required to be more positively powered or less negatively powered than the distance correction. Some presbyopic persons have both near vision and distance vision defects, requiring simultaneous or alternating vision lenses to properly correct their vision.
Simultaneous vision lenses refers to the class of bifocal or multifocal contact lenses in which optical power for distance vision and near vision are positioned simultaneously within the pupil area of a user's eye. They are generally composed of, within the pupil area of the eye, two or more concentric annular zones which alternately provide the distance and near power, or a multifocal zone having an aspheric surface which provides a continuous gradient of optical power over a selected range of powers. The visual performance of the simultaneous vision lens design is limited by its dependence on pupil size. Moreover, with all simultaneous vision lenses a partially degraded image of an object is projected onto the retina. The consequence of this image degradation is a reduction in visual actuity and/or contrast sensitivity (less signal, more noise), and the quality of the degraded image may or may not be acceptable to the patient. The clinical effects of this degradation may be measured objectively in terms of reduced visual acuity and contrast sensitivity. The subjective effects of the degradation are perceived by the patient in various ways which are collectively referred to as subjective blur. Therefore, when wearing a simultaneous vision lens, the patient may not be selecting between separate distance and near images. Rather, in the presence of subjective blur the patient may be attempting to function with the reduced level of spatial information content that is provided by a degraded image.
Alternating vision refers to the class of segmented (or translating) bifocal contact lenses in which the lens is divided into two optical zones. Typically the superior (or upper) zone is for distance vision correction, whereas the lower zone is for near vision correction. The distance portion (upper zone) subtends the pupil of the eye in primary gaze, while in downward gaze, the add power or near portion (lower zone) of the lens subtends the pupil. Effective use of an alternating vision lens requires vertical translation of the optical zones across the pupil when the eye changes from primary gaze to a downward gaze. In such a situation, the lens must move such that the pupil is predominately subtended by the distance zone for primary gaze and predominately subtended by the near zone for down-gaze. Unlike the simultaneous vision lenses, the visual performance of the alternating vision lenses is not significantly limited by its dependence on pupil size.
Moreover, large population of presbyopes also has an astigmatism vision error. Those presbyopes may require to wear contact lenses capable of correcting both astigmatism and presbyopia. Such contact lenses are disclosed in a co-pending U.S. patent application Ser. No. 60/400,254 filed Jul. 31, 2002 (incorporated herein by reference in its entirety) and have a cylindrical optical surface (or power) to correct astigmatism vision errors and a multifocal power to compensate for presbyopia.
Human eyes may suffer not only low-order monochromatic aberrations, such as defocus, astigmatism and prism, but also high-order monochromatic aberrations, such as a non-standard amount of spherical aberration, coma, and other irregular high-order aberrations. High order aberrations present in an eye can blur images formed on the retina, which can impair vision. The impact of these higher-order aberrations on retinal image quality can become significant in some cases, for example, in older eyes, in normal eyes with large pupils, and in the eyes of many people with irregular astigmatism, keratoconus, corneal dystrophies, post penetrating keratoplasty, scarring from ulcerative keratitis, corneal trauma with and without surgical repair, and sub-optimal outcome following refractive surgery. For those people, visual acuity of 20/20 or better can be achieved with customized contact lenses or contact lenses capable of correcting high-order monochromatic aberrations of the human eye. Unlike current contact lenses, customized contact lenses or contact lenses capable of correcting high order aberrations inevitably need to have a complex surface design without restrictions of rotational symmetry.
Effective use of each of the above described contact lenses and the like requires a rotational and/or orientational stability on an eye. One of the most commonly utilized basic techniques, which can maintain a predetermined orientation of a contact lens on an eye, is prism stabilization. By using this technique, a contact lens is provided with prism ballast, which is generally a base-down prism to increases the mass of the lower portion of the lens and to create a weighting effect to orient the lens. However, there are some disadvantages associated with designs of contact lenses having prism ballast in the prior art. Those lenses are not comfortable to wear and/or not highly effective in maintaining lens orientation. Furthermore, contact lenses with prism ballast as orientation feature may not be able to provide better visual performance since they tend to have optical distortions known as prism.
There are other orientation/stabilization features which have been used or proposed to be used in contact lenses. Exemplary orientation/stabilization features include a faceted surface in which parts of the lens geometry is removed to control the lens orientation and double slab-off features which have a top slab-off zone and a bottom slab-off zone zones to maintain the lens orientation on the eye as well as a visual cue on the lenses so that a wearer can insert the lenses in the proper orientation. The lenses with these orientation/stabilization features are not comfortable to wear and/or not highly effective in maintaining lens orientation.
Therefore, there is a need for a contact lens with an orientation feature, wherein the lens is substantially free of prism optical effects and is comfortable to wear.
An object of the invention is to provide a contact lens having an orientation feature which can maintain effectively the lens in a desired rotational and orientation on an eye, wherein the lens is substantially free of prism optical effects and is comfortable to wear.
Another object of the invention is to provide a method for producing a contact lens having an orientation feature which can maintain a predetermined orientation on an eye, wherein the lens is substantially free of prism optical effects and is comfortable to wear.